Troubleshooting
Line Rebalancing Procedure
Summary:
Equipment
Required:
Step 1.
Step 2.
Step 3.
Step 4.
Step 5.
Step 6.
Step 7.
Step 8.
Cross talk sometimes occurs between pairs in a long multiple pair cable, such as those used
to connect key phones located several hundred feet from the equipment rack. This may
indicate the need to obtain better than average balance on these lines. The following proce-
dure can be used to accomplish this and should eliminate the problem.
High impedance (1 mcgohm min.) AC meter with minimum 10 mv resolution.
T
WO
1OOk resistors, matched within 1%. Capacitor, .005 uf., 47k resistor.
Resistance decade box, or substitution box covering the range 1 k to 15 k ohms.
Resistors to install for rebalancing.
Refer to
Drawing KM 0899,
build the circuit shown from the parts listed above, and use it on
each line to be tested. Do not connect the Decade Box until instructed. If only measuring %
Unbalance, go to Step 3.
If rebalancing lines, turn TCIV
Power Off
and remove the resistors, marked R2 from the LLM
line circuits of all lines to be rebalanced. Then turn the
Power On.
Using a test phone, call the first line to be rebalanced, establish communication, and leave
both phones off hook.
Hold clown the three bottom buttons on the test phone to send a 941
Hz
tone to the line
under test.
Note: Do not press any tone buttons on the line being rebalanced, as this would result in
abnormal d.c. and damage the accuracy of the rebalancing.
Measure the tone level from terminal
T
to ground
(Vt)
and the voltage from terminal
R
to
ground (Vr). TCIV lines may operate 10 to 30 % unbalanced; however, rebalancing can
reduce this to less than 2%. The unbalance is determined using the following formula:
U% =
100 x (Vt-Vr)/(Vt +Vr)
Note:
If low level noise causes difficulty when measuring near null (0) voltages, release the
tone button to see how low the meter dips and use this as the new 0
level.
If the noise is fluc-
tuating, set it to the lowest average value.
Connect the
Decade Box as
shown in the Line
Balancing Set-up
figure on the following
page.
Adjust the Decade Box until the meter reads close to 0 millivolts. Note the setting of the
Decade Box for the line being tested; then, remove the test circuit and repeat Steps 3 through
7 for each line to bc rebalanced,
Turn the
TCN
Power Off; then, using the Decade Box setting noted for each line, install a
standard resistor of the value noted in place of the removed R2 using one of the following
methods: When finished, turn the TCIV Power On.
Internally in place of R2; This requires careful soldering and de-soldering on the LLM
and the resistor will have to be changed again if the line hybrid (Ul) is changed.
Externally by wrapping one lead to the
T
pin and soldering or wire-nutting the other
end to pin 71 (+ 12vdc).
Rauland-Borg Corporation
(Orig. 10/88; Rev. l/89)
Page
25 of 30
